Electrical sliding contact



May 16, 1933. J. -n5 1,908,908

ELECTRICAL SLIDING CONTACT Filed June 10, 1952 IN VENTOR 7 f TORNEY ng the mercury,

Patented May 16, 1933 HOMER J. LOFTIS, OF IRON TON, OHIO, ASSIGNOR PATE FICE TO HENRITE PRODUCTS CORPORATION,

CF IRON'ION, OHIO, A CORPORATION OF OHIO ELECTRICAL SLIDING CONTACT Application filed June 10, 1832.

This invention relates, generally, to electrical contacts; and the invention has reference, more particularly, to a novel sliding electrical contact.

Heretofore, sliding electrical contacts, such as those used in variable rheostats, potentiometers and other electrical apparatus, usually consist of pieces or shoes of metal Which are pressed against the resistor body and are adapted to be' slid thereover, to thereby vary the effective resistance of the rheostat or other apparatus. In order to insure a good electrical engagement between the contact piece and the resistor body, the contact piece is applied with a firm pressure to the resistor body, resulting in the rapid wear of these members. Also, more or less arcing usually takes place in use, resulting in the burning and deterioration of the sliding contacts. These sliding contacts usually have a relatively high contact resistance which is objectionable, necessitating the use of relatively large and expensive contacts where the currents to be handled are appreciable. V

The principal object of the present invention is to'provide a novel electrical sliding contact employing mercury as the contacting medium.

Another lies in the sliding object of the present invention provision of a novel electrical contact having a receptacle for holdthe said receptacle having a contacting portion provided with an opening therein, whereby the mercury is caused :to directly engage the object, such as the resistor, cooperating with the contact.

Still another object of the present invention is to provide a novel electrical sliding contact which is of extremely simple construction and is adapted to be applied with an extremely light pressure to the resistor or other member swept over by the contact, thereby substantially eliminating wear of these members, the said novel sliding contact providing a highly eflicient electrical connection with a very low ohmic resistance and having an exceedingly long life in practice.

Other objects of this invention, not at this Serial No. 616,408.

time more particularly enumerated, will be clearly understood from the following detailed description of the same.

The invention is clearly illustrated in the accompanying drawing, in which:

Fig. 1 is a perspective view of the novel sliding contact of this invention illustrated as applied to a rhcostat.

Fig. 2 is a sectional View taken substantially along line 22 of Fig. 1.

F ig. 3 is a plan view of the structure showii in Fig. 2; and

Figs. 4 and 5 are fragmentary sectional views of modified forms of sliding contacts.

Similar characters of reference are employed in all of the above described views to indicate corresponding parts.

Referring now to Figs. 1 to 3 of said drawing, the reference numeral 1 designates a typical rheostat, such as the type used in radio circuits, which rheostat is provided with the novel sliding contact of this invention. Rheostat 2 consists of a disk shaped base having a C-shaped resistor body 2 mounted thereon. Resistor body 2 may be made of any suitable electrically resistant material. This resistor body may be molded, for example, from a mixture of electrically resistant and electrically conducting materials held together by a suitable binder. Thus, feldspar and mica may be used as the electrically resistant materials, carbon black or graphite as conducting materials and a condensation product may be used as the binder. 85

The rheostat 1 is provided with a central operating shaft 3, which shaft has a reduced extension 5 extending through a central aperture provided in the disk shaped base of therhcostat. A threaded bushing 4, of electrically conducting material, surrounds the reduced extension 5 of shaft 3. The upper portion of bushing 4 is illustrated as enlarged at 4 for engaging the upper surface of the rheostat base. A nut 6 is threaded upon the bushing 1 and engages the under surface of the rheostat base, thereby locking the bushing 4 to such base. A contact strip 7 is illust as gripped between the rheostat base and nut (5 to thereby provide an 1 electrical connection between the operating shaft 3 and an exterior circuit. Contact strips 8 and 9 are connected to the respective ends of the resistor body 2 and serve to connect these ends to an external circuit.

A resilient contact carrying arm 10 is secured to the upper end of the reduced extension 5 of shaft 3 as by means of a screw 11. The contact carrying arm 10, is preferably formed from a piece of sheet metal having the shape especially illustrated in Fig. 3, which piece of sheet metal is formed with a C-shaped slot 12 therein providing a substantially circular portion 13 for at- 15 tachment to the operating shaft 3. The outer end portion of the contact carrying arm 10 has a button or projection 14 secured to the under surface thereof as by means of a screw 18, extending through an aperture in the contact arm and threaded into the butat all times. Owing to the conical shape of the portion 15, the same acts to hold the entire under surface of the washer member 16 uniformly in contact with the surface of resistor body 2. The washer member 16 is preferably amalgamated, i. e. the same is treated with mercury so that at least the outer portion thereof becomes amalgamated with the mercury, thereby preventing the absorption of liquid mercury within the interior of this washer member by the member itself. The resilient arm 10 acts through button 14 to hold washer 16 with a light pressure against the surface of resistor body 2. The washer member 16 is filled with liquid mercury 17 which serves as the con tacting medium between the sliding contact and the resistor body 2.

In use, to vary the effective ohmic resistance of the rheostat 1, it is merely necessary to turn operating shaft 3, thereby causing contact carrying arm 10, acting through button 14, to move washer 16 and its contained mercury over the surface. of the resistor body 2. The mercury 17 within the washer member 16 makes a perfect contactwith re- 60 sistor body 2 and with the conical portion 15 of button 14, even though an extremely light pressure is applied by spring arm 10 to the contact washer 16. Thus, the ohmic resistance of the novel sliding contact of this 65 invention is very low, and the same may be used for carrying relatively large currents without sparking or heating.

It will be apparent that the novel sliding contact of this invention may be made in various forms other than that shown in Figs. 1 to 3 of the drawing. Thus, in Figs. 4 and 5, modified forms of this sliding contact are illustrated. In Fig. 4 a hollow cylindrical contact member 19 is illustrated as attached directly to the end portions of spring arm 10. The bottom of contact member 19 is open so that the mercury 18 therein will engage directly against the surface of resistor body 2. A screw 20 is illustrated as extending through an aperture in contact carrying arm 10 and is threaded into an aperture provided in the closed top of contact member 19, thereby attaching this contact member to the contact arm.

In Fig. 5 the resilient contact carrying arm 10 isillustrated as carrying a substan tially semi-spherical hollow contact member 21. The curved outer surface of the contact member 21 enables the contact carrying arm 10 to rock somewhat on the contact member 21 so that this contact member will automatically accommodate or align itself to the surface 2 of the resistor body. A screw 22 is illustrated as employed for attaching contact member 22 to the arm 10.

Although the novel sliding contact of this invention is illustrated as used in connection with a rheostat, it is to be understood that the same ma be used wherever a sliding contact is desired, the same enabling a positive electrical connection to be obtained with but little pressure exerted between the contacting surfaces, and thereby substantially eliminating frictional wear. Owing to the relatively great surface tension of the liquid mercury 17, the same tends to remain within the hollow contact member and will last an exceedingly long time in use. After a long period of service, if additional mercury is desired to be inserted into the contact member, as shown in Figs. 1 to 3, it is merely necessary to raise the outer end of the contact carrying arm somewhat and to insert the additional mercury into the interior of the contact member 16. In the forms of the invention shown in Figs. 1 and 5, it is merely necessary to remove the holding screws 20 and 22, whereupon additional mercury may be inserted into the interior of the contact members through the apertures thus provided. Although these contact members are shown as moving over horizontal surfaces,- it is to be "understood that the same will operate just as readily over vertical surfaces while still retaining the mercury therewithin.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from sense.

the scope thereof, as defined by the following claims, it is intended that all matter contained in the above ,description or shown in the'accompanying drawing shall be interpreted as illustrative and not in a limiting What is claimed is:

1. In combination, a body and an electrical contact slidable relative to said body, said electrical contact comprising, a hollow contact member having a portion thereof slidably engaging said body, said portion having an aperture therein, and liquid mercury contained within said hollow member and contacting with said body by Way of said aperture.

2. In combination, a body having a contacting surface and an electrical contact slidable with respect to said surface, said electrical contact comprising, a hollow memher having an open mouth portion abutting the contacting surface of said body, means for holding said hollow member so that the open mouth portion thereof will fit snugly against the contacting surface of said body,

and liquid mercury contained within said hollow member and retained therein by the contacting surface of said body.

3. In c0mbination, a body having a contacting surface portion and an electrical contact slidable over said surface portion, said electrical contact comprising, a hollow member having an open end thereof abutting the contacting surface portion of said body, ,resilient means for holding said hollow 6. In combination, an annular resistor body having a flat contacting surface, an electrical contact slidable with respect to said surface, an operating shaft turnable withinsaid resistor body, a resilient contact arm carried by said operating shaft and engaging said contact to hold the same in operative relation with said contacting surface, said contact comprising a holder member having an open mouth portion engaging said contacting surface and liquid mercury contained within said holder member.

In testimony, that I claim the invention set forth above I have hereunto set my hand this 20th day of May 1932.

HOMER J. LOFTIS.

member so that said open end thereof isresiliently pressed against said contacting surface portion, and liquid mercury contained within said hollow member and directl engaging the containing surface portion 0 said ody.

4. In combination, a body having a uni- .form contact surface and an electrical contact slidably engaging said surface, said electrical contact being hollow and having an. opening therein adjoining said contact surface, liquid mercury contained within said contact, said mercury communicating with said contact surface 'throu h said opening, and a resilient arm for hohfi and for moving said contact over said sur- H face.

conical button in coo said washer member in 5. In combination, a body having a flat contact surface and an electrical contact slidable over said surface, said electrical contact having one side said contact surface,

contained within said washer member.

ng said con-' tact in engagement with said contact surfaceand liquid mercury 

